1. Field of the Invention
The present invention relates to a document reading apparatus and a method of calculating a position adjustment amount thereof.
2. Description of the Related Art
A document reading apparatus is known, wherein a document placed on an original platen is scanned and an image of the document is read through a minification optical system. The document reading apparatus has a scanning section for scanning the document. This scanning section has a light source for illuminating the document, and a reflection mirror for reflecting light reflected from a surface of the document to guide it to an imaging sensor. By this document reading apparatus, the light reflected by the reflection mirror is focused on the imaging sensor via a focusing lens.
Usually, the scanning section is constituted of independent two units. A first unit has the light source and one reflection mirror (first mirror). The first unit moves in one direction (in a sub-scanning direction) along the original platen. The light source illuminates the document and the first mirror reflects light reflected from the document in a moving direction of the unit. Therefore, the first mirror is supported at an angle of 45 degrees with respect to the document surface. A second unit has two reflection mirrors (second mirror and third mirror), with each reflective surface arranged at an angle of 90 degrees to each other, whereby the light reflected from the first mirror is reflected below an incident light and in a direction opposite from an incident direction of the incident light. The second unit moves in the same direction as the first unit, whereby the light reflected in the sub-scanning direction by the first unit is reflected in an angular transversal U-shape. The light reflected by the second unit forms an image of the document on the imaging sensor through the focusing lens, with the optical axis along the aforementioned moving direction.
The imaging sensor and the focusing lens do not move with respect to the document during document scanning. The second unit moves at half the speed with respect to the first unit. Each unit is pulled and driven by a driving wire directly fixed to each of the units or hung on the unit through a pulley. By designing an extending manner of this driving wire, the ratio of a moving speed between the first unit and the second unit is maintained to ½. Accordingly, an optical path length from a reading position of the document to the imaging sensor is maintained constant, irrespective of a position of the scanning section. The first to third mirrors and the focusing lens constitute the aforementioned minification optical system.
As the imaging sensor, a linear image sensor for reading the image in one direction (main scanning direction) as a plurality of pixels is used. The imaging sensor is attached so as to read the image of the document in a width direction orthogonal to the sub-scanning direction, namely in a length direction of the original platen.
The focusing lens is adjusted and fixed, so that its optical axis is parallel to the length direction of the original platen. This is because the position in the width direction is not allowed to be deviated, when the scanning section is positioned at a tip end portion of the document and at a rear end portion of the document. Namely, the position, where both edges in the width direction of a rectangular document are read, is adjusted so as to be constant wherever the scanning section may move. More specifically, the direction of the optical axis is adjusted in such a way that when the vicinity of the tip end of the rectangular document is read, 100 numbered pixel and 5140 numbered pixel of the image sensor are at the positions of both edges, and when the vicinity of the rear end of the document is read, 100 numbered pixel and 5140 numbered pixel of the image sensor are also at the positions of both edges.
Meanwhile, reading of the document has not been considered to be influenced by a deviation of the scanning section in the width direction. As described above, the scanning section has the light source and the reflection mirror. There is almost no change in light distribution in the width direction of the light source, namely in the main-scanning direction, with respect to a minute deviation of about several pixels. In addition, even when the reflection mirror deviates in the width direction, the optical path length is unchanged. However, when the reflective angle changes by a twist, or the reflection mirror is displaced due to vibration in a direction other than the width direction, the reading of the document is adversely influenced thereby. For example, when there is a distortion in attachment or driving of the driving wire, thereby a stress in the width direction is accumulated, the twist and the vibration are thereby caused. Therefore, a known document reading apparatus is constituted in such a way that a play is provided in a rotation axis direction of the pulley on which the driving wire is hung, to allow the pulley to move freely in the width direction (for example, see Japanese Laid-Open Patent Publication No. 08-95167).
Conventionally, it is so considered that even when the scanning section is deviated in the width direction, an image quality is not influenced thereby. However, it is found that conditions differ when a dust corresponding to a pixel width or larger than the pixel width is adhered to the first mirror.
The first mirror is located at a position close to the document in the optical path of the minification optical system. Even if this position is off the depth of focus of the focusing lens, the dust adhered to the first mirror is read as a blurred image. When shading correction is performed, with such a dust adhered to the first mirror, a reading sensitivity of each pixel of the image sensor is adjusted so that a read result of the reference white board including the blurred image of the dust is uniform. Namely, the pixel with the image of the dust is adjusted to a higher sensitivity than an original state, based on the read result which is darker than the original reference white board. Here, the reference white board serves as an image for adjustment for correcting variation of sensitivity of each pixel of the imaging sensor and of light distribution of the light source. The reference white board is provided in the vicinity of the tip end portion or rear end portion of the original platen.
When the moving direction of the scanning section is coincident with the optical axis of the focusing lens, the pixel for reading the image of the dust is unchanged, whichever position in the sub-scanning direction the scanning section is located. A higher sensitivity is set for this pixel based on the shading correction, and therefore the influence of the dust is corrected. However, when the moving direction of the scanning section is deviated from the optical axis of the focusing lens, the pixel for reading the image of the dust is deviated to the adjacent pixel as the scanning section gets away from the reference white board. When this deviation quantity exceeds the width of one pixel, a streak of light and darkness appears in the read image.
As a method of preventing such a malfunction, adhesion of the dust to the first mirror is eliminated. However, a technique to completely shield the inside of the image reading apparatus is not a practical way in terms of cost. Even if the first mirror is cleaned regularly, it cannot help being affected by the dust adhered thereto before next cleaning.
As another method, the sub-scanning direction, namely, the moving direction of the scanning section, is made coincident with the direction of the optical axis of the focusing lens. In the conventional document reading apparatus, a particular mechanism therefor is not provided.